Build server's Debian version update

Hello there, I am studying F-Droid build infrastructure in the scope of a thesis related to Reproducible Builds. In particular I’m trying to understand how the build server VM is managed when it comes to a Debian upgrade. I noticed that recently build server was upgraded to Debian Trixie from the commit history and I was interested to track when these upgrades have been done in time. Is the version update announced somewhere or should I rely only on commit date? From when do the apps start to be built using the latest Debian version? Is the change immediate? Also, I have other questions about the infrastructure and reproducible builds management, could you please point me to the right channel to have them answered? Thank you so much!